[PATCH] libstdc++: simd: allow x86 -mlong-double-64

[Jonathan Wakely]

On Tue, 24 Mar 2026 at 06:30, Alexandre Oliva <oliva@adacore.com> wrote:
>
>
> On a target that defaults to -mlong-double-64 -msse,
> standard_abi_usable.cc and other tests fail to compile because of the
> assert that rejects long double.  As on ppc, we can trivially make
> things work for -mlong-double-64.
>
> Regstrapped on x86_64-linux-gnu.  Also tested with -mlong-double-64 on
> libstdc++ tests that triggered the static_assert.  Ok to install?
>
>
> for  libstdc++-v3/ChangeLog
>
>         * include/experimental/bits/simd.h (__intrinsic_type): Accept
>         64-bit long doubles on x86 SSE.
> ---
>  libstdc++-v3/include/experimental/bits/simd.h |    4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
>
> diff --git a/libstdc++-v3/include/experimental/bits/simd.h b/libstdc++-v3/include/experimental/bits/simd.h
> index c2ac5adedaccf..b96818af6e6ff 100644
> --- a/libstdc++-v3/include/experimental/bits/simd.h
> +++ b/libstdc++-v3/include/experimental/bits/simd.h
> @@ -2469,7 +2469,9 @@ template <>
>  template <typename _Tp, size_t _Bytes>
>    struct __intrinsic_type<_Tp, _Bytes, enable_if_t<__is_vectorizable_v<_Tp> && _Bytes <= 64>>
>    {
> -    static_assert(!is_same_v<_Tp, long double>,
> +    // allow _Tp == long double with -mlong-double-64
> +    static_assert(!(is_same_v<_Tp, long double>
> +                   && sizeof(long double) > sizeof(double)),
>                   "no __intrinsic_type support for long double on x86");


Would this be a little easier to understand as:

if constexpr (sizeof(long double) > sizeof(double))
  static_assert(!is_same_v<_Tp, long double>, "...");

>
>      static constexpr size_t _S_VBytes = _Bytes <= 16 ? 16 : _Bytes <= 32 ? 32 : 64;
>
>
> --
> Alexandre Oliva, happy hacker            https://blog.lx.oliva.nom.br/
> Free Software Activist     FSFLA co-founder     GNU Toolchain Engineer
> More tolerance and less prejudice are key for inclusion and diversity.
> Excluding neuro-others for not behaving ""normal"" is *not* inclusive!